How large is the tank? Chromis do better in groups. These groups should usually be in odd numbers, obviously no less than 3. With that said, don't be surprised if you slowly see the numbers dwindling. It seems to be the chromis' way.

I have a school of about 20. They do great in a big school. They dwindle in numbers because they starve. They have a very short gut and cannot store much food reserves, so they need to eat a little several times a day. And in a school this becomes even more important as the smaller fish lose out to the bigger ones. JMO